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So if you want to know why bitcoin is falling, it's because large bitcoin holders are using FIFO instead of average method for their taxes. Give it another month and it might be about $800, and still people will keep posting these threads, not understanding the tax consequences.
so basically you are saying, that there are few people with a cristall ball, who know exactly, where the price will be at a given time and than they sell and buy to avoid(optimize) taxing....
The taxing issue is very new and mostly for US citizens who, according to some are not the major holders (unlike China, Japan) so I doubt it is about taxes now.
It is still the echo of the bloody Gox and the chineese uncertainty, as many said before.